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  1. GBA Editor
    Martin Holladay | | #1

    Answers to both of your questions can be found here: Do I Need a Vapor Retarder?

    Q. "What are the classes of vapor retarders?"

    A. Class III vapor retarders have a perm rating between 1.0 perm and 10 perms. These materials aren't vapor barriers, but they slow down the flow of water vapor somewhat. Examples include stucco, one or two coats of latex paint, 1 inch of EPS foam insulation, and more than 5 inches of open-cell spray foam. (Note that the greater the thickness of a piece of foam insulation, the lower its permeance.)

    Class II vapor retarders have a perm rating between 0.1 perm and 1.0 perm. These materials slow down the flow of water vapor to a greater extent than materials that are considered Class III vapor retarders. Examples include plywood, OSB, the kraft facing on fiberglass batts, 1-inch-thick XPS foam insulation, and one coat of vapor-retarder paint applied to drywall.

    The most impermeable materials are called “Class I vapor retarders” or “vapor barriers.” There materials include glass, sheet metal, aluminum foil, and polyethylene.

    Q. Where are vapor retarders placed?

    A. The most important location to install a vapor barrier is under a concrete slab. Beyond that recommendation, there is no rule.

    In most climates, you don't want to install polyethylene on the interior of your walls or ceilings. Vapor barriers are sometimes useful for stopping inward solar vapor drive, especially if the vapor barrier is a layer of thick rigid foam. In some very cold climates, interior vapor retarders (or a "smart" vapor retarder) is useful. For more information, see the article I linked to.

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